Fusion

1
Fusion

• Powers the sun
• In fusion, we combine two atoms and release
  energy
• Easiest to do this with H or its isotopes
• We already talked about the proton-proton chain
  in the sun
• D-T reaction-takes deuterium and tritium and
  creates He

2
Fusion

• In order for these reactions to occur, one needs
  the deuterium and tritium at high temperatures
• For the DT reaction, T 40 x 106 K
• For DD reaction, T 100 x 106 K
• DT is good for bombs, not so good for long term
  power, and tritium has a half life of 12 years

3
Thermonuclear devices

• Also called H bombs, these are fusion bombs
• Require fission to compress and heat the fusion
  fuel.
• The fusion releases enormous amounts of high
  speed neutrons which are then often used to
  induce fission in matter that it is normally
  difficult to induce fission in (such as depleted
  Uranium, Uranium composed mostly of 238U).
• This adds to the radioactive fallout of the bomb

4
Fusion reactors

• Main problem is maintaining the fusion material
  at high enough T so that fusion produces the bulk
  of the energy (break even)
• Confined plasma via a magnetic field so it does
  not contact the container walls (which would cool
  it and quench the reaction)
• Not nearly as hazardous as fission reactors, most
  studies show severe failures to be contained
  within the plants themselves.

5
Barriers to fusion power

• Scientific feasibility of the reactions
• Economics
• Damage to reactor components due to large flux of
  high energy neutrons
• Availability of materials to build the reactors
• Maybe in 50 years -but we have been saying that
  for 50 years!

6
Planes, trains and automobiles

• 27 of the total national energy budget goes into
  transportation
• Of this 27, 35 is used by automobiles
• Autos are among the least energy efficient modes
  of transportation (Bicycles are number 1)
• Rely in the internal combustion engine

7
What does it take to move a car?

• Four force terms need to be considered
• Force needed to accelerate the vehicle
• Fa ma
• Force needed to climb any hills
• Fhmsg, where s is the slope of the hill
• Force needed to overcome internal energy losses
  (tire flexure, wheel bearings, friction with the
  road surface, etc)
• Fr Crmv, where Cr is a constant term
• Force needed to overcome aerodynamic drag on the
  vehicle, depends upon speed.
• Fad CD Af v2 /370 where CD is the aerodynamic
  drag coefficient, Af is the frontal area of the
  vehicle.
• So the total force required is the sum of these 4
  terms
• FT Fa Fh Fr Fad

8
Energy required

• The energy required will be equal to the work
  done by the force over a given distance or
• E W Fd or
• E Pt, where P is the power output and t is the
  time the vehicle is operated or
• E Fvt
• So to minimize energy, you need to minimize the
  forces.

9
Making current cars more efficient

• Minimize the force required
• mamsg CrmvCD Af v2 /370
• Make m small
• Make Cr small
• Make CD small
• Make Af small
• Make v small
• Or any combination of reducing these values

10
Alternatives to the internal combustion engine

• Flywheels
• Electric batteries
• Hybrids
• Alcohol
• Hydrogen

11
Flywheels

• Energy storage device
• Flywheel is spun up and the energy is stored as
  rotational energy to be used at a later time
• Designed to resist losses of rotational energy
  due to friction, etc
• Energy stored is given by
• Ek I?2
• where I moment of inertial of the
  flywheel, and ? is the angular velocity.
• The moment of inertial is a function of the mass
  and the distance from the center of rotation
• So the structure of the flywheel and the
  rotational rate determine the amount of energy
  stored.
• Ultimate limit on the energy storage is the
  strength of the flywheel. Spin it too fast, and
  it will tear itself apart.

12
Flywheel vehicles

• Could extract energy from braking-rather than
  waste the energy into frictional heating of
  brakepads, reverse the engine and spin up the
  flywheel.
• Need to be recharged on the power gird, saves
  gas, but drains electricity
• The big implementation problem is materials which
  can withstand the stress needed to spin the
  flywheel fast enough to make this a worthwhile
  alternative.
• Prototype mass transportation vehicles have been
  built (In Sweden and by Lockheed)
• Used in Formula 1 racing to recover energy lost
  in braking and along with a continuously variable
  transmission to improve Formula one car
  acceleration.
• Also used in the incredible hulk roller coaster
  at Universal Islands of Adventure in Orlando,
  Fl.
• Ride starts with an uphill acceleration, rather
  than a gravity drop.
• Flywheels are used to provide the initial energy
  impulse, otherwise the park would brown out the
  local energy grid everytime the ride began.

13
Hybrids

• Still use gasoline powered engines, but combine
  them with (usually) batteries to achieve better
  fuel economy.
• Idea is to use as small as possible a gasoline
  engine, and only when it can be run at peak
  efficiency.
• Use excess power to recharge the battery (no need
  to tap the power grid)
• Use energy from braking (regenerative braking) to
  also charge the battery
• Work best in stop and go driving.
• Major initiative in the auto industry right now.
• Result in using less gas-stretching our fossil
  fuels

14
Pure electric vehicles

• Powered by an electric motor, rather than a
  gasoline engine
• Needs batteries current generation of batteries
  have 520 times less energy density than gasoline.
• Need to be charged from the power grid
• If all the vehicles in the US were converted to
  electric cars, it would triple the current
  electric energy generation
• Recharging electric vehicles takes time- several
  hours, whereas it takes minutes to refill your
  gas tank
• Batteries have a finite lifetime, need to be
  replaced every 2-3 years at a current cost of
  1000
• Limited range (less than 100 miles before
  recharging is needed)
• Ultimate limit is current battery
  technology-current lead acid batteries have not
  changed much in 100 years.
• Environmental effects from the disposal of lead
  acid batteries
• No new promising battery technologies on the
  horizon to substantially help electric cars

15
Fuel cells

• An electrochemical conversion device
• Chemical reactions cause electrons (current) to
  flow
• Requires a fuel, an oxidant and an electrolyte (
  a substance that contains free ions and acts as a
  conductor)
• Typical type of fuel cell is called a proton
  exchange membrane fuel cell (PEMFC)